1. Field of Invention
This invention relates to diagnostic electronic equipment, specially to finding out defective capacitors in a circuit.
2. Description of Prior Art
Detecting defective parts right in a circuit reduces consumption of time and cancel necessity of repeating harmful soldering. The major problem of this diagnostic technology is to eliminate an influence of surrounding circuitry which distorts reading of measuring apparatus.
Inventors created several types of diagnostic techniques and apparatus that have much in common. U.S. Pat. No. 4,216,424 to Vette et al. (1980) discloses Method and apparatus for testing electrolytic capacitors in-circuit based on employing a low voltage AC ohmmeter to measure Equivalent Series Resistance (ESR) at relatively high frequency 100 KHz that makes a capacitive reactance Xc of the test capacitor negligible so measuring current mostly flows through the capacitor is being tested. Portion of measuring apparatus serves to protect the meter from damaging by remaining DC charge.
U.S. Pat. No. 5,159,526 (1992) to Marek et al. shows Method and apparatus for determining orientation of polarized capacitors in-circuit by means of applying signals to inner and outer conductive plates of capacitor with a further analyzing of external electric field generated by the outer plate. Although being in-circuit, this method covers only limited characteristic (orientation) and requires extended (3) number of terminals being connected to the capacitor that makes it inconvenient. U.S. Pat. No. 5,502,375 (1996) also to Marek et al. discloses modification of this method reducing to two repeated measurements revercing the voltage and reference signals. The ratio of magnitudes of electric field in both position of measuring electrodes determines the orientation. This method requires an additional time and still does not measure quality-related characteristics of capacitors.
The group of methods for a non-destructive measurement of conductive and non-conductive thin layers can be also related to in-circuit capacitive methods in terms of physical processes. U.S. Pat. No. 4,968,947 (1990) to Thorn et al. and U.S. Pat. No. 3,801,900 (1974) to Szasz show measuring systems for such layers based on evaluating a displacement current through measuring arrangement with layers to be measured. These methods and apparatus can not be used for discrete elements with capacitive characteristics.
All recited methods and apparatuses suffer from a number of disadvantages:
(a) The necessity to connect at least 2 measuring electrodes to leads of capacitor is being tested. In real electronic boards capacitors' terminals have a limited access and coated with isolating layer with a purpose of security. It makes diagnostic process inconvenient and time-consuming. PA1 (b) The quality of conventional in-circuit testing depends on probing frequency. The more frequency, the less an influence of surrounding parts because a high-frequency current mostly flows through capacitor is being tested rather than through surrounding circuitry with non-capacitive characteristics. This excludes possibility to measure at low frequencies when necessary and limits the scope of in-circuit methods. PA1 (c) The known methods suffer from impossibility to evaluate in-circuit one of two connected in-parallel capacitors. Measuring current simply flows through both of them, that drastically reduces the resolution of in-circuit methods. PA1 (d) The use of ESR only also limits an informational value of in-circuit methods because ESR by itself does not define quality of capacitor uniquely. Quality of capacitor is defined by its dielectric loss related to its tangent of dielectric loss angle tg.delta.=.omega..multidot.C.multidot.ESR, where .omega. is an angular frequency and C is capacitance. Quality factor of capacitor Q=1/tg.delta., so two capacitors with equal ESRs can have distinct qualities, depending on capacitance. PA1 (e) The conventional apparatus suffer from complexity because of special measures to reduce the influence of electric environment: parallel circuitry, non-linear two-terminal remaining charges and so on. PA1 (a) to provide measuring with one measuring electrode connected with no necessity to connect it to capacitors' leads or terminals; PA1 (b) to provide measuring in a wide range of frequencies, depending on necessity and therefore increase in-circuit methods' informational value; PA1 (c) to provide possibility evaluate one of two connected in-parallel capacitors without influence from another; PA1 (d) to provide increasing informational value of the method by measuring quality factor Q and tangent of dielectric loss angle tg.delta. of capacitor in-circuit; PA1 (e) to provide more reliability of measuring apparatus by means of reducing number of parts.